1. Field of the Invention
The present invention relates to a hydraulic braking system for use in an automotive vehicle, and more particularly to a hydraulic braking system which has a hydraulic power pressure source for pressurizing a brake fluid by a fluid pump to generate a hydraulic power pressure.
2. Description of the Prior Art
A conventional service braking system for an automotive vehicle is provided with a hydraulic braking pressure generator for applying a hydraulic braking pressure to wheel brake cylinders mounted on road wheels in response to depression of a brake pedal. The hydraulic braking pressure generator includes a static hydraulic pressure generator such as a master cylinder which introduces a brake fluid from a reservoir and generates a hydraulic braking pressure in response to depression of the brake pedal, and a dynamic hydraulic pressure generator which has a power pressure source for generating a hydraulic power pressure and regulates the hydraulic power pressure supplied therefrom to generate a hydraulic braking pressure regulated in response to depression of the brake pedal.
As to the dynamic hydraulic pressure generator, there is known a hydraulic booster which actuates the master cylinder with the hydraulic power pressure supplied from the power pressure source in response to depression of the brake pedal. U.S. Pat. No. 4,565,411 discloses the hydraulic booster which is provided in the hydraulic braking system as the dynamic hydraulic pressure generator in addition to the master cylinder. In other words, a hydraulic pressure generated by the hydraulic booster in response to depression of the brake pedal is applied directly to the wheel brake cylinders in order to reduce the stroke of the brake pedal. According to the hydraulic braking system as described above, the road wheels are prevented from being locked in braking operation by suitably controlling the hydraulic braking pressure applied to the wheel brake cylinders by means of valve devices, and the acceleration slip of driven wheels is prevented in starting or accelerating operation by communicating the wheel brake cylinders directly with the power pressure source to apply the braking force to the driven wheels.
In U.S. Pat. No. 4,848,853, granted Jul. 18, 1989, there is proposed a system in which in the antilocking operation, the hydraulic pressure is applied from the hydraulic booster to the wheel brake cylinders to which normally the hydraulic braking pressure is applied from the master cylinder, while in the anti-slip operation, the hydraulic power pressure is applied to the wheel brake cylinders through a power pressure changeover valve. Namely, in the anti-locking operation, the hydraulic pressure applied to the wheel brake cylinders is changed over by a first changeover valve to the hydraulic pressure supplied from the dynamic hydraulic pressure generator, while in the anti-slip operation, the wheel brake cylinders are communicated with the power pressure source by a second changeover valve. Thus, the second changeover valve functions as the power pressure changeover valve. With these first and second changeover valves operated in sequence, the smooth changeover operation is ensured without applying the hydraulic power pressure to the master cylinder when the wheel brake cylinders are communicated with the power pressure source.
The above-described hydraulic braking system is so arranged that the hydraulic power pressure supplied from the power pressure source is controlled to a value within a predetermined range by intermittently actuating an electric motor for driving a fluid pump. However, when the fluid pump continues to be driven for some reason, the hydraulic power pressure is increased to exceed a predetermined upper limit value. The continuous output of such a high hydraulic power pressure causes a trouble of the fluid pump or the like. Therefore, the conventional hydraulic braking system is so arranged that the power pressure source is provided with a relief valve which communicates the power pressure source with a reservoir when the hydraulic power pressure supplied from the power pressure source exceeds a certain pressure for opening the relief valve. However, the value of the pressure for opening the relief valve has large dispersion, and it is not easy to maintain it within a certain range.